ABSTRACT This ancillary study will recruit subjects as they are simultaneously enrolled into the ongoing randomized placebo-controlled study, The American Trial using Tranexamic Acid in Thrombocytopenia (`A-TREAT'; NCT02578901). The goals of the parent A-TREAT study are to evaluate whether the antifibrinolytic agent tranexamic acid improves the hemostatic efficacy of prophylactic platelet transfusion, and reduces the use of platelet and red cell transfusions with an acceptable safety profile in patients with severe hypoproliferative thrombocytopenia following chemotherapy or stem cell transplantation. Plasma samples obtained from study subjects will be used to determine whether a `hyperfibrinolytic' plasma profile can be identified using novel global assays of fibrinolysis, and whether this profile is associated with higher bleeding rates. The results of this study will not only address mechanisms of bleeding (in addition to thrombocytopenia) in severely thrombocytopenic patients, but will also suggest a potential biomarker of bleeding risk and/or the benefit of tranexamic acid in individual patients undergoing myeloablative therapy. Furthermore, given the intense recent interest in the role of tranexamic acid in a variety of other inherited and acquired bleeding disorders, we anticipate that these studies will also be highly relevant to a greater understanding of pathophysiological mechanisms and/or the development of a useful biomarker in other clinical scenarios.